Individual study: Survival of semaphore cactus Opuntia corallicola in relation to planting proximity to prickly pear Opuntia stricta (a cactus moth Cactoblastis cactorum host plant) at Saddlebunch Keys, Florida, USA
Stiling P., Moon D. & Gordon D. (2004) Endangered cactus restoration: mitigating the non-target effects of a biological control agent (Cactoblastis cactorum) in Florida. Restoration Ecology, 12, 605-610
The natural distribution of the cactus moth Cactoblastis cactorum is Argentina, Paraguay, Uruguay and southern Brazil. It was released as a biological control in the 1950s and 1960s to several Caribbean islands to control Opuntia spp. cacti. The first US record of cactus moth came from Florida in 1989. It was thought that the moth had either dispersed from Cuba or had been brought in on cacti imported from Haiti. In the USA it now threatens native cacti and concern is greatest for the endangered semaphore cactus, Opuntia corallicola, of which only two wild populations exist. Planting trials were therefore undertaken in an attempt to bolster semaphore cacti numbers and to test the effectiveness of different planting treatments to protect it from cactus moth. In this study, whether semaphore cacti planted near prickly pear O.stricta (a common cactus which hosts cactus moth) would be attacked by Cactoblastis and, if so, whether they would survive, was tested.
Study site: Whether semaphore cactus Opuntia corallicola planted near prickly pear O.stricta cacti would be attacked by cactus moth Cactoblastis cactorum, and the effect of planting in the sun versus shade on cactus growth, nitrogen levels, attack by cactus moth and other mortality was tested on the Saddlebunch Keys, Florida, southeast USA. This 'Associational Susceptibility Experiment' ran from 1998 to 2003.
Experimental design: In 1998, 180 small (< 30 cm tall) semaphore cacti (cultivated, from wild collected cactus pads, at the University of South Florida) were divided into 18 replicates of 10 plants each, and planted in two rows of five, 20 cm between plants, with 50–100 m between each replicate. Eight replicates were planted in the shade of buttonwood trees Cornocarpus erectus and 10 in full sunlight. Six of the replicates were planted within 5 m of prickly pear infested with Cactoblastis, six within 5 m of non-infested prickly pear, and six between 20 - 30 m from any prickly pear. Cacti were surveyed every 3 months for 5 years, until 2003. Height and mortality were recorded. After 5 years, average height of the surviving cacti in each replicate was used to compare growth between sun and shade treatments. Plant quality was estimated by removing one small pad from one of the cacti in each replicate after 2 years of growth and analyzing for nitrogen.
Causes & characteristics of semaphore cacti mortality : Semaphore cacti killed by Cactoblastis exhibited a hollow appearance as all tissues between the epidermal layers were removed, usually characteristic frass (droppings) or live caterpillars within cacti were also present. Many plants also rotted at the crown, with the fungi Fusarium oxysporum present in the rotted crown area. A few cacti were knocked over, uprooted or destroyed by falling branches.
Semaphore cacti mortality rates: In total, 128 of our 180 planted cacti died over the 5 years of the experiment (total mortality = 71.1%). The cacti died at a fairly constant rate of between 14 to 15% per year. The greatest source of mortality was from Cactoblastis, which killed significantly more cacti in shaded conditions (68%) than in full sun (28%). There was no effect of prickly pear proximity on attack rates. The only other cause of mortality in the experiment was crown rot, though crown rot did not significantly differ between treatments (see Table 1, attached). Semaphore cacti did not grow significantly more in the shade (18.2 cm ± 14.4 SE) than in the sun (12.0 cm ± 8.3). Plant nitrogen content was significantly higher in the shade (%N = 1.34 ± 0.06) than in the sun (%N = 1.22 ± 0.07).
Conclusions: Semaphore cacti planted between 20-30 m away from the prickly pear (which act as a reservoir for cactus moth) were just as frequently attacked and killed by Cactoblastis as semaphore cacti planted within 5 m. In addition, Cactoblastis attack was greater in the shade than in the sun, possibly because shaded plants had higher nitrogen content.
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